1. Field of the Invention
The present invention relates to a recording material for the production of offset printing plates, which includes a metal substrate on whose front a photosensitive layer is present and on whose back a layer of an organic polymeric material having a glass transition temperature T.sub.g of 45.degree. C. or more is present.
2. Description of the Related Art
Recording materials for the production of offset printing plates are usually delivered in stacks of 20 or more. After a relatively long storage time, at high pressure and/or elevated temperature, the individual plates frequently stick to one another and scratches may form in the photosensitive layer, on the back of the substrate during transport, or on removal of the plates from the stack. To prevent this, paper sheets are usually placed between the plates. In in-line packing systems, the interleaf paper is typically automatically inserted between the plates. This step is relatively slow and also susceptible to errors.
Such interleaves are particularly necessary in the case of recording materials having an aluminum substrate without a backing coating. The paper can, however, have an undesirable effect on the photosensitive layer and change it. For example, components of the paper can produce a change in the pH and in the photosensitivity and/or cause more rapid aging of the photosensitive layer. With surface-sealed papers, the problem can be reduced. However, such papers are substantially more expensive.
In the printing works, the plate stacks are processed on automatic lines in turn. The interleaf paper is typically removed by blowing the paper out from between the stacks. This reduces processing time and is susceptible to faults. Moreover, the interleaf paper cannot be re-used and must be disposed of as waste. According to Japanese Patent Publication No. A 2-040,657, this problem is minimized for a recording material having an aluminum substrate, which comprises a photosensitive layer on one side and a UV-cured layer of photopolymerizable material on the other side. The layer applied to the back may also contain photosensitizers, thermal polymerization inhibitors, binders, fillers and other additives in addition to the monomers. This layer may also act as a dulling layer.
Japanese Patent Publication A 6-202,312 discloses a recording material for the production of offset printing plates. The aluminum substrate of the recording material is coated on the back with an organic polymer, such as, for example, polyethylene, polypropylene, polybutadiene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polystyrene or a methacrylate resin. According to the publication, the attack by aqueous alkaline developer on the aluminum substrate is reduced by the presence of the backing coating. The photosensitive layer in the recording material contains from 1 to 10% by weight of a compound insoluble in the developer.
Japanese Patent Publication A 9-265,176 discloses a recording material having an anodized aluminum substrate, a photopolymerizable layer on the aluminum oxide layer produced by the anodization and a 0.1 to 8.0 .mu.m thick backing coating. The coating includes a saturated copolymerized polyester resin, a phenoxy resin, a polyvinyl acetal or a vinylidene chloride copolymer, each having a glass transition temperature T.sub.g of 20.degree. C. or more. According to the publication, scratching of the plates during transport in the stack as well as delamination of the radiation-sensitive layer as a result of excessively strong adhesion to the back of the plate present on top are to be prevented.
European Patent Publication No. A 528,395 describes a recording material for the production of offset printing plates. The material can be stacked without interleaf paper. The recording material includes an aluminum substrate with a 0.01 to 8.0 .mu.m thick layer of an organic polymeric material having a glass transition temperature of not less than 20.degree. C. on the back of the substrate and a photosensitive layer on the front of the substrate. A discontinuous dulling layer which comprises particles having a mean diameter of not more than 100 .mu.m and an average height of not more than 10 .mu.m is formed on the photosensitive layer. The weight of the dulling layer is from 5 to 200 mg per square meter. The dulling layer aids the removal of air between photographic negative and photosensitive layer in the vacuum contact printing frame. The dulling layer can be produced, for example, by spraying a solution of a methyl methacrylate/ethyl acrylate/acrylic acid terpolymer, some of whose carboxyl groups are present in salt form, on the photosensitive layer in an electrostatic field with the aid of an atomizing bell rotating at about 25,000 rpm. In general, the dulling layer is soluble in water or aqueous alkali. However, dulling layers, in particular those of a material having a low glass transition temperature, tend to stick to the back of the plate present on top in the stack. Relatively large parts of the radiation-sensitive layer can thus become detached, so that the recording material may no longer be used.
European Patent Publication No. A 490,515 relates to a presensitized printing plate which, after imagewise exposure, is developed with an aqueous alkali metal silicate solution. To prevent developer from dissolving aluminum from the back of the plate, the back of the plate is provided with an organic polymeric coating which is insoluble in the developer. The coating contains polymers, such as polyethylene, polypropylene, polybutene, polybutadiene, polyamide, polyurethane, polyurea, polyimide, polysiloxane, polycarbonate, epoxy resins, polyvinyl chloride, polyvinylidene chloride or polystyrene. The polymer may also comprise a heat-curing or photochemically curing component.